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    Real-Time Imaging of Crystallization in Polylactide Enantiomeric Monolayers at the Air-Water Interface

    Year: 2008

    Journal: Langmuir, 2008, 24 (19), pp 10791–10796, 20111221

    Authors: Young Shin Kim, Christopher M. Snively, Yujuan Liu, John F. Rabolt and D. Bruce Chase

    Organizations: Department of Materials Science and Engineering, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716, and Central Research and Development, DuPont Experimental Station, Wilmington, Delaware 19880-0328

    A newly developed planar array infrared reflection−absorption spectrograph (PA-IRRAS) offers significant advantages over conventional approaches including fast acquisition speed, excellent compensation for water vapor, and an excellent capacity for large infrared accessories, e.g., a water trough. In this study, the origin of stereocomplexation in a polylactide enantiomeric monolayer at the air−water interface was investigated using PA-IRRAS. PA-IRRAS was used as a probe to follow the real-time conformational changes associated with intermolecular interactions of polymer chains during the compression of the monolayers. It was found that a mixture of poly(d-lactic acid) (PDLA) and poly(l-lactic acid) (PLLA) (D/L) formed a stereocomplex when the two-dimensional monolayer developed at the air−water interface before film compression, indicating that there is no direct correlation between film compression and stereocomplexation. PA-IRRAS spectra of the stereocomplex exhibited distinct band shifts in crystalline sensitive components, e.g., the vas(C−O−C, h) mode, as well as amorphous-dependent components, e.g., the vs(C−O−C) mode, when compared with the spectra of PLLA alone. On the other hand, time-resolved PA-IRRAS spectra, which were obtained as the films were being compressed, revealed that both monolayers of PLLA and mixed PLLA/PDLA stereocomplex were crystallized into a 103-helix and a 31-helix, respectively, with a distinct band shift in crystalline sensitive components only. Fourier self-deconvolution of the spectra demonstrated that the band shift in crystalline sensitive components is correlated with the intermolecular interaction of polymer chains.